A standard subscriber telephone transmits and receives over two or a pair of wires known as a subscriber line or loop. The data or voice is an analog voltage signal across the two wires and is known as VF. Status of the subscriber's telephone, such as Off/On Hook, Hook flash, Disconnect and Dial Pulsing is also transmitted over the two wires, as a combination of voltage and current signals and are known as DC signalling. The voltage and current signals or DC signalling are for status signalling and are sufficiently different from the voice signals so as not to be confused and unnecessarily interfere. The two wires from the subscriber's telephone are connected on the other end to a central office. The central office monitors the subscriber's line for the status or state of the subscriber's telephone. Depending on the status of the subscriber's telephone, the central office takes appropriate action. If the subscriber's telephone is Off Hook, then the central office will be prepared to receive Dial Pulsing and connect the subscriber's line to another subscriber's line so that voice signals can be exchanged between the two subscribers.
The standard analog telephone system represents signalling for loop-supervision which hasn't electrically changed in almost 100 years. All advances in the switching of telephone traffic had to be backwards compatible to the installed base of telephones.
Older telephone systems used saturable core reactors, line feed transforms, and other magnetic devices to monitor the Loop for Off/On Hook, Hook Flash, Disconnect and Dial Pulsing. The advantage of the magnetic device was its natural integration of noise spikes out of the signalling. The disadvantage of the magnetic device was its size.
Current Telephone Line-Card technology no longer employs magnetic devices in the loop for signal supervision; instead ASIC-mixed technologies and digital processing techniques are used to power the line and monitor loop current for the DC-Loop signalling. The Line Card has lost its natural ability to integrate out noise spikes as a result of not using a magnetic device for Line Feed.
Modern telephone equipment typically use fast computers for call processing. A Call-Processing State Machine is implemented in today's fast micro processors. Such a State Machine operates fast compared to the older relay-logic based machines. Thus, will react to noise pulses in the DC Signalling from the line card.